Vinyl ester based polymer latex compositions, and especially vinyl acetate/ethylene based polymer latex compositions are well known in the art. Such compositions have been made and used for decades as adhesives, especially for paper and packaging, wood glues, in paints, coatings and inks, for treating leather, or for bonding fabrics, especially non-woven fabrics.
The rapid increase in sales of disposable non-woven products in the past has intensified interest in improving emulsion polymers used to bind these non-wovens. Special emphasis has been put on wet tensile strength of the bonded non-woven product, since this is an important factor in consumer acceptance and utility. Such non-wovens include for example diapers, sanitary napkins, medical gowns, wipes, tabletops or other products requiring a certain tensile strength in the wet state.
Emulsion copolymers prepared from vinyl acetate, ethylene and a crosslinker, mainly N-methylol acrylamide, when applied to an non-woven substrate, often exhibit unacceptable loss in strength in the presence of water and organic solvents. They further often have unacceptably short shelf life, or suffer from lack of latex stability. They may further provide insufficient adhesion strength to a substrate. Such shortcomings limit the use of these formulations in many commercial and industrial applications. Consequently, the non-wovens industry has been searching for a latex composition used as a binder which can overcome these limitations.
Numerous formulations have been developed in an effort to overcome the above problems associated with vinyl acetate/ethylene (VAE) emulsions. The most successful approaches use N-methylol acrylamide as a co-monomer for crosslinking vinyl acetate and ethylene. Colloidal stability of the resulting dispersion has been addressed by using surfactants and/or protective colloids during emulsion polymerization of vinyl acetate and ethylene. Examples of this art include US patent application publication 2003/0, 176, 133; U.S. Pat. Nos. 3,380,851; 3,345,318; 3,440,200; 4,332,850; and 5,109,063; German laid open publications 25 12 589; 42 40 731 (EP 672,073 corresponds); 44 32 945 (U.S. Pat. No. 5,763,033 corresponds); or 196 31 935 (U.S. Pat. No. 6,174,568 corresponds); and European patent application 0 609 849.
The use of certain surfactants in such compositions, especially when used in formulations of paints, coatings, inks, adhesives or for bonding nonwovens has recently been identified as a possible cause of environmental problems. Several attempts have thus been undertaken to avoid the use of surfactants for stabilizing vinyl acetate based polymer latex compositions. In many cases stabilization is provided by a protective colloid instead of or in addition to the surfactant. Examples of corresponding art include German laid open publications 41 13 839 (CA 2066988 corresponds); 102 04 234; 40 32 096 (CA 2053005 corresponds); and 198 21 774; European patent applications 1 392 744 (US patent application publication 2004-0132939 corresponds); and 0 727 441 (U.S. Pat. No. 5,889,101 corresponds); and U.S. Pat. Nos. 4,440,897; 4,316,830; 6,117,960; 4,745,025; and 3,954,724. The use of a protective colloid adds complexity to the system, increases costs, and may cause a deterioration of other desirable properties. For instance it may cause yellowing of a bonded nonwoven.
In view of the above it is the object of the present invention to provide a vinyl acetate based polymer latex which is substantially free of a protective colloid, which is also substantially free of a surfactant or comprises only minor amounts of the same. It is further an object of the present invention to provide a vinyl ester based polymer latex which, when used for bonding non-wovens, allows for improved wet tensile strength of the bonded product.